The invention relates to a method for checking the plausibility of the signals of a tank filling level sensor of a motor vehicle. The subject matter of the present invention also comprises a computer program and a computer program product with a program code which is stored on a machine-readable carrier, for carrying out the method.
Filling levels of liquids, for example tank filling levels in vehicles, are sensed by tank filling level sensors. In this context, a differentiation is made between tank filling level sensors which carry out continuous, as it were “analog”, sensing of the filling level, and filling level sensors which merely sense whether or not liquid is present at a signal generator position, referred to as “discrete” filling level sensors. The former are used, for example, in fuel tanks for vehicles, the latter are preferably used in, for example, supplementary tanks, for example of SCR systems. Existing and future regulations prescribe ever more stringent monitoring of exhaust-gas-related components. For example, in the case of exhaust-gas-related sensors it is therefore necessary to carry out plausibility tests even on filling level sensors, on the basis of which tests it is possible to decide whether or not the respective sensor is functionally capable. In the case of sensors which supply just one discrete signal in the above sense, a second sensor is often installed for the purpose of monitoring safety-critical components.
In modern diesel vehicles, what is referred to as SCR technology is used. This includes the selective catalytic reduction of nitrogen oxides in the exhaust gases of internal combustion engines as well as other combustion installations. The chemical reaction of reduction is selective here. This means that not all exhaust gas components are reduced but rather only the nitrogen oxides (NO, NO2). For the reaction to occur, ammonia is required, which is added to the exhaust gas. The products of the reaction are water and nitrogen. In the case of vehicles, the required ammonia is not used in a pure form but rather in the form of an aqueous urea solution, which is known by the trade name “AdBlue”. The solution is injected into the exhaust gas train upstream of the SCR catalytic converter, for example by means of a metering pump or an injector. Ammonia and water are produced from the urea/water solution by means of a hydrolytic reaction. The ammonia which is generated in the process can react with the nitrogen oxides in the exhaust gas in a special SCR catalytic converter at an appropriate temperature. The quantity of injected urea is dependent on the (motor engine) nitrogen oxide emission and therefore on the instantaneous rotational speed and the torque of the engine. The consumption of urea/water solution is, depending on the raw emission of the engine, approximately 2 to 8% of the consumed diesel fuel. For this reason, a tank with urea/water solution must be installed in the vehicle, and the filling level in this tank has to be sensed. The tank filling level which is sensed by the filling level signal generator may be falsified here depending on driving states, but also depending on the profile of a section of road. For example, when the vehicle travels on a severe positive gradient or a severe negative gradient an incorrect filling level value may be output by the tank filling level signal generator. In view of the stringent exhaust gas regulations which require, for example, that automobile manufacturers cause the driving capability of a vehicle to be severely restricted if the exhaust gas limiting values can no longer be complied with, it is desirable to sense the tank filling level precisely, independently of the driving state or of the profile of a section of road.